A 56-year-old Florida man was charged Friday in the nationwide mail-bomb scare targeting prominent Democrats. Experts had predicted that forensic evidence left behind by the bomb maker would help law enforcement track down a suspect. Here are some of the issues involved:

EXAMINING THE DEVICES

The innards of the devices—the type of pipe used, the filler and the type of mechanism designed to set it off—offered all sorts of clues. Besides any genetic material that the alleged bomber left behind, the materials themselves often point investigators toward who made and sent the explosives.

FBI Director Christopher Wray said Cesar Sayoc's fingerprints and possible DNA were collected from two of the 13 devices. Wray said the fingerprints matched a print found on one of the packages sent to Rep. Maxine Waters of California.

OTHER PHYSICAL EVIDENCE

Before advances in DNA, other types of physical evidence often helped authorities. But even something as small as a stray hair can help identify a suspect.

David Chipman, a retired ATF agent and explosives expert who is now a senior adviser at the Giffords Law Center to Prevent Gun Violence, recalled working on a case in Texas in which a dog hair found on electrical tape on the device helped prove who made the bomb.

“It will be a treasure trove of forensic evidence,” said Anthony Roman, a private security and investigations consultant. “As human beings, we are filtering off our DNA everywhere we walk, everywhere we sit.”

Experts said DNA or fingerprint evidence does not necessarily steer authorities directly to the perpetrator, but is used to verify that the suspect they have identified is responsible.

ANALYSIS OF MATERIALS USED

The bombs seized were made using 6 inches of PVC pipe, a small clock, a battery, wiring and “energetic material.” Previously, law enforcement officials said they also contained shards of broken glass and were wrapped with black tape.

The use of broken glass and PVC pipe could point to the bomb maker wanting to ensure the devices were as light as possible to avoid shipping restrictions, Chipman said.

Broken glass as filler would be lighter than nails or other metal, and a PVC pipe would be lighter than a metal pipe. The postal service requires packages weighing more than 13 ounces to be shipped from a retail counter, and it returns any heavier packages that are dropped into a mailbox or slot.

The federal criminal complaint charging Sayoc said that each of the devices was packaged in a manila envelope lined with bubble wrap. Each had about six postage stamps bearing a picture of an American flag.